1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, especially, to a supertwisted nematic liquid crystal display apparatus (STN-LCD) capable of a large volume and superfine display by using a multiplex driving means.
2. Description of the Prior Art
The liquid crystal display apparatus of the above described type comprises a pair of facing base plates having electrodes, a liquid crystal layer including supertwisted nematic liquid crystal, a pair of polarizers, and a driving means capable of charging at least two different voltages. The facing base plates hold the liquid crystal layer between, and the polarizers have a structure being arranged outside the facing base plates.
A STN-LCD becomes possible to display a large volume and superfine display driven with a multiplex driving means by (i) increasing 90 degrees twisted angle of conventional twisted nematic liquid crystal display to 180 degrees, and (ii) using a birefringent mode having 45 degrees for an angle forming by transmitting axis of the polarizer and a direction of an orientating process of the base plate.
However, as a side reaction of the above (ii), problems such as coloring of display, and lowering of a contrast ratio were caused. The contrast ratio is expressed by a factor, (transmission ratio in a bright state/transmission ratio in a dark state), and as a result of the side reaction, transmission ratio in a dark state increases and consequently, the contrast ratio is lowered.
A phase plate type STN-LCD which comprises the STN-LCD added with a birefringent plastic film resolved almost all of the coloring problem of the display, but the contrast ratio was at most about 20:1.
Additionally, various countermeasures have been proposed in JP-A-2-37321 (1990), JP-A-2-67518 (1990), JP-A-3-13916 (1991), JP-A-3-13917 (1991), JP-A-4-190325 (1992), JP-A-4-215619 (1992), and K. KUMAGAI et al (JAPAN DISPLAY 1989 page 312-315).
The prior art disclosed in JP-A-2-37321 (1990) intended to enhance a contrast ratio of the phase plate type STN-LCD by coinciding a wave length dependency of birefringence of the phase plate with that of a liquid crystal composition. However, as a twisted nematic layer has a twisted structure, an optical characteristics of the twisted nematic layer differs entirely from that of a bulk liquid crystal composition which does not have any twisted structure. Accordingly, the optical characteristics required for the phase plate is not only identical to that of the bulk liquid crystal compound, and the above prior art can not realize a higher contrast ratio than that of the present state.
The prior art disclosed in JP-A-2-67518 (1990) used two kinds of resins, the one has a positive oriented birefringence and the other has a negative oriented birefringence, respectively. Because of having a steep threshold characteristics, an optical characteristics the phase plate type STN-LCD changes quickly depending on alteration of charged'voltage. Furthermore, the optical characteristics of the phase plate type STN-LCD has a wave length dependency, and the wave length dependency also changes quickly depending on alteration of the charged voltage. Accordingly, a display characteristics of the phase plate type STN-LCD changes remarkably depending on an arrangement of the phase plate and the polarizer, a driving condition of the STN-LCD, the optical characteristics of the phase plate, etc. The prior art disclosed in JP-A-2-67518 (1990) does not make optimization of the above described various condition. It is impossible to realize enhancing of contrast ratio of the phase plate type STN-LCD only by changing materials of the phase plate and increasing numbers of the phase plates.
In the prior art disclosed in JP-A-3-13916 (1991), a laminated phase plate wherein two phase plates having different wavelength dependency of birefringence each other are laminated so that each of extended axises crosses perpendicularly is used for the phase plate type STN-LCD. However, in contrast with the birefringent media used in JP-A-3-13916 (1991) which has a retardation of about 600 nm at wavelength 550 nm, the wavelength dependency of birefringence which is defined by an equation (2) explained later is 0.32. It is impossible to obtain a more enhanced contrast ratio than that of the present art using the above described birefringent media.
In the prior art disclosed in JP-A-3-13917 (1991), a laminated phase plate wherein two phase plates having different wavelength dependency of birefringence each other are laminated so that each of extended axises are placed in parallel is used for the phase plate type STN-LCD. However, the same problem as the one described above regarding to the art disclosed in JP-A-3-13916 (1991) can be said.
In the prior art published in "JAPAN DISPLAY" (1989) pp. 312-315, K. KUMAGAWA discloses a contrast ratio improving method for a STN-LCD using two phase plates. Polarization state of transmitting light with each wavelength distributes in a manner drawing an arc on a Poincare sphere sighted from a S.sub.3 axis direction. Therefore, the contrast ratio is enhanced by converting the distribution to a distribution on a line using a first phase plate, subsequently, focusing the distribution on a line on a point of an equator on the Poincare sphere using a second phase plate.
The above described method can be practical if a wavelength dependency of birefringence of the phase plates can be controlled arbitrarily. However, actually, the birefringence of the phase plate has a restricted wavelength dependency. Therefore, the birefringence of the phase plate can not be controlled arbitrarily. Accordingly, regarding to the first phase plate, it is impossible to convert the distribution of the polarizing state of the transmitting lights with respective wavelengths to a complete linear state. Regarding to the second phase plate, it is impossible to concentrate the distribution on a line on a point of the equator.
Furthermore, K. KUMAGAWA did not teach any of optical characteristics of a liquid crystal layer, particularly on its applied voltage dependence.
The prior art disclosed in each of JP-A-4-190325 (1992) and JP-A-4-215619 (1992) has the same content as that of K. KUMAGAWA, and accordingly, both the prior art have the same problem.